Nowadays healthcare relationships rest on the recognition of the right of patients to take part in the decisions about their health. This idea is the foundation of the informed consent theory. Nevertheless, problem arises when the patient cannot take part in these decisions because he hasn't enough capacity. Then, another person, a substitute, must decide for him. In Spain does not exist enough clarity about the criteria that must guide the decisions of the proxy. The present work deals with the three criteria developed by the North American jurisprudence. These criteria are: the subjective criterion, the criterion of the substitute judgment and the best interest or major benefit criterion. The subjective criterion is based on the statement of the own patient, usually written in an advance directive or living will. The criterion of the substitute judgment tries to rebuild the decision that the own patient had taken if he remained capable. The criterion of the major benefit tries to protect the well-being of the patient. Traditionally the «better interest» has been to defend the life at any expense, without attending to another type of considerations. Probably it is the moment to look for a new consensus on what today the society has to consider the «better interest» of a patient. Surely this new definition meaning would not stem exclusively from the right to life, but from the conjunction between quantity and quality of life considerations and the freedom of patients, all interpreted in the light of the respect person's dignity.